cli: fix warnings with -Wswitch-default

mmcli-output.c: In function ‘output_item_free’:
  mmcli-output.c:321:5: error: switch missing default case [-Werror=switch-default]
    321 |     switch (item->type) {
        |     ^~~~~~
  mmcli-output.c: In function ‘mmcli_output_dump’:
  mmcli-output.c:1208:5: error: switch missing default case [-Werror=switch-default]
   1208 |     switch (selected_type) {
        |     ^~~~~~
  mmcli-output.c: In function ‘mmcli_output_list_dump’:
  mmcli-output.c:1231:5: error: switch missing default case [-Werror=switch-default]
   1231 |     switch (selected_type) {
        |     ^~~~~~

cli/mmcli-common.c

@@ -1551,10 +1551,9 @@ mmcli_get_state_reason_string (MMModemStateChangeReason reason)
         return "Suspend";
     case MM_MODEM_STATE_CHANGE_REASON_FAILURE:
         return "Failure";
+    default:
+        g_assert_not_reached ();
     }
-
-    g_warn_if_reached ();
-    return NULL;
 }
 
 /* Common options */